High temperature alloy steel



. I and which are ductile at high Application February 12 1957 Serial No. 639,644

llClaims. (c1. 1s+1z6 No Drawing.

This invention relates to an alloy steel capable of being age-hardened, having good corrosion resistance, particularly at elevated temperature, and a long stress rup ture life with good ductility at elevated temperatures, and to articles made therefrom.

'l'he alloy steel of the present invention is suited for various other uses,butis particularly adapted to meet the, ever increasing requirements demanded of turbine parts such asblades, buekets, bolts, etc, Such parts have been subjected to higher audhigheroperating tempera tures and frequently are operated today at temperatures in the range of 800 to 1400 F. To be suitable for such use, the turbine parts must, rosion, resistance at these temperatures and because they are subjected to high pressures at these high temperatures, must also have as desirable a stress rupture life as possible. In addition, such an alloy should not become of course, have good cor UnitedStates Paten 0" ICC,

Other objects of the invention will be apparent to those skilled in the art from reading this specification.

We have found that an alloy can be prepared which meets all of these demands described above to a high degree by incorporating in a 12% chromium steel (i. e., containing about 946.5% chromium) certain proportions of molybdenum or tungsten (i. e., about 647%). By proper adjustment and control of this composition, a product can be made which .after'age hardening is essentially ferritic, which has excellent stress rupture characteristics at elevated temperatures, e. g. at 1200" to 1400 E, which is of relatively low cost to manufacture and which can be annealed soft enough for easy machining. For example, such an alloy can be prepared so that it is readily machineable when annealed but can beeasily aged to a Rockwell C hardness of above 25. The same alloy, when given a stress rupture test under a 50,000 p. s. i. load at 1200 R, will have a lifeof atleast about hours and frequently as high as 1150 hours. titanium, vanadium and boron may be added to enhance the properties of the basic alloy. l

The composition which has been found'to be capable of producing these results is'las follows:

c ,Percent Carbon T 0.10 Manganese 1.0 Silicon 1.0 Chromium o o o r e 9-l6.5 Molybdenum and/or tungsten 6-17 embrittled but should be ductile andret'ain its ductility,

under these operating conditions for long periods of time; it should be capable of being annealed so that it can be easily machined atroom temperature; and it should be made of reasonably low cost ingredients. Also, the alloy should have a coefficient of expansion hich approximates that of the parts to which the alloy is attached'fsuch as the housing, rotors, etc. 3 In orderto'acliieveas many of these properties aspossible,,a1lo'y steels have heretofore been used containing.

tofore been necessary toyresort" to thehighly alloyed austen-itic alloys, containing largeamouuts of nickel, cobalt, or arnonuts of chromium well above, 12%. These alloys are extremely expensive, are ditficult to fabricate and haye a substantially higher coefiicient of expansion alloys are attachedz-- Itfis an object of thisinvention ablei inexpensive, essentially' ferriticalloys-of the 12% chromium steel class, which contain a precipitate in the. structure when "age hardened, capable of withstanding considerable stress and of Thigh" corrosion resistance to, 6

content represents the desirablemaximum limit. We

within the steam and the "atmosphere at temperatures up to..1400 temperatures.

It is another object of this invention to provide inexpensive alloys of the 12% chromiumsteel class, which are essentially ferritic in structure, hardened toa value of aboveRoclt'well (I -25 tomachiningwith a minimum of' 'distortion.

aound 12% chromium," along with smaller, amounts tha'riith'e parts to which the components made of these toprovide age-harden capable of being age subsequent Balance substantially all iron, except for incidental ime purities in such steels.

ltdaxlmum.

desirably limited to 0.1% maximum to insure an essen tially ferritic microstructure. It is preferably limited to 0.05%. In the. present alloy, carbon is not required for strength. By limiting the carbon to 0.05%, the chromium used for chromium carbide formation is minimized. This increases the ainount of chromium available for corrosion resistance and for its effect in insuring an essentially ferritic micro structure. p q The manganese and silicon contents are desirably limited to 1% maximum. These elements are present solely to provide for deoxidation duringrnelting. Manganese tends to produce an austeniticmicrostructurei and for this reason is desirably limited to 1% maximum,

Chromium is necessary in the alloy of the invention for corrosion resistance and it has been established that 9% is required for thi s purposes Also below a content of 9% chromium, stress rupture properties are not satis factory. As chromium content is increased, at the 10% molybdenum level, stress rupture life increasesv very markedly up to 11.3% chromium and then gradually decreases with further additions up to about 16.5 chromium. Our evaluation based on corrosion resistance. and stress rupture life indicates that a 16.5% chromium prefer to maintain the chromium content range of between about 10 *and14%.

Phosphorus is added for its known beneficial effects a c A in strengthening ferrite and improving the stress rupture; properties. This improvement can be effected within tbei r l nted Feb. 24, 1959 I Optional elements including nickel, phosphorus,

this range the addition of phosphorus does not provide a proportionately beneficialincrease in stress rupture properties. Phorphorus, -has the adverse effect of lowering stress 'rupturgJductility; butinickel can be added to, overcome this--deficiency. 1

The basic properties of the alloyr'are substantiallyimproved by the use'of;6l7% molybdenum or -tungsten. We have found thatthis controlled amount of molyb-- denum or tungsten provides an alloy with an essentially ferritic microstructure which surprisingly has excellent resistance to stress ruptureat elevated temperatures. Ferrite normally :is very soft and ductile, and :ferritic steels have notbeen considered to be "good high'temperature.;al loys by those skilled in the art. p

We have discovered that the presence of molybdenum and tungsten either separately in amounts of about 6% and above, or together in amounts totaling about 6% and above, improve the stress rupture properties after aging-from the annealedpondition. These stress rupture properties are much higherthan in any other alloy ;of the ferritic class known to us. The aging reaction .provides hardness and strength atelevated temperature, the amount and effect of which-is largely controlled by the molybdenum or tungsten content. It is desirable to have a minimum aged hardnessof Rockwell C25. 6% molybdenum will providethe, minimum :required aged hardness of Rockwell C-25. Amounts of molybdenum above 1 4% produce increasingly large quantities of one or more intermetallic compounds so'that it is difiicult to forge the "alloys; "Additions of molybdenum and/or tungsten over "17%increase the hardness and brittleness due to the increase in the amountofintermetallic com:

pounds so as to render the alloy impractical. The preferred range of molybdenum and/or tungsten is between about 8 and 14%.

Nickel may also be employed in the alloys of the invention. Nickel is added to provide improved stress ruptured ductility. The maximum amount of nickel to be added will vary with the chromium and molybdenum or tungsten'additions. 'The basic alloy is an essentially ferritic alloy and additions" of nickel'can change it to an austenitic alloy. With lower additions of molybdenum or tungsten and chromium such as each, 4% nickel can be added and still provide an essentially ferritic microstructure. With larger amounts-of chromium and molybdenum, the amount'ofnickel can be raised to a maximum of "6% when the concentrations of chromium and molybdenumjor 'tungsten are at the maximum perc'entage' claimed, and still provide aiwholly ferritic alloy.

Inorder that our 'inventionniay be better under; stood, the Iollowing example of alloy steels prepared according to this invention are given for the purposes of illustration. I Percent concentrations of elements in the examples and throughout thespecifications are intended to refer to percent b'y weight, in secs-dam with the general'practice in this art. I H I Example I. An alloy was "prepared having the following analysis: 7

7 Percent The alloy, for testpurposes, was cast into 1% in. square ingots weighing,800 gramseach and forged to a /2 in. square bar. The sample showed an essentially t'er'ritic microstructure when annealed by oil quenching from 00 -F. and aged at F200 for 16 hours and:

had a -Rockwell C hardness a: 47/48. Samples were i l ifllge ot betweenabout,0.10%.and 0.30%, as above gamma v g site subjected last-res asters til-a are recorded in the following table:

Percent Percent Test Temp. Load, Elongation Reduc Life in p. 5.1. in one-inch tion in Hours gauge length Area 60,000 (.0 7.6 179.2 so, 000 a. s s. a 1, 152.2 15, 000 8.4 25. 8 -68 '7 Example Il.-An alloy was prepared having the @01- lowing analysis:

j e'ct'ed to stress rupture tests arid-the results recorded below; j g

. firs-cent l lfercent Test Temp. 'Lo'sd, Elongation Redu'e- Life in pL-Js i. 21110116411011. -tlon-tn Hours".

ao,ooof m 55.1 7 1?..0

Example Illa-An alloy-was prepared having the 01 lowing analysis: I

v Percent Carbon -022; Manganese I 0.-30 Silicon 0- 1-3 Chromium 098 Molybdenum 10.64 Nickel JD D Phosphor'us Titanium 7 0.04

Iron

This alloy Was.;subiected"to 1 same" treatment Example @I. It then had ,a Rockwell .C hardness of;

48/49. This -alloy as agejd was *essentially :ferritic in; structure. When subjected to stress prupmre tests at? 1200 F. under load of 60,000 p. s. 1.,thesamples had 1 life of 94.2 hours and showed a.4.0% =elongation in oneinch gauge length with a reduction inarea 064.7%. 1

Example V.An alloy waspreparedghaving the fol- When subjected to the same'treatmentas specified in Example I, the product was essentially ferritic in structure after annealing and aging. Samples were subjected to stress rupture tests at 1200 F. at aload of 60,000 p. s. i. and had a life of48.3 hours. t j

Example VI.-An alloy was prepared having the following analysis:

Percent Carbon 0.022 Manganese 0.38 Silicon 0.32 Chromium 11.46 Molybdenum 6.8 Phosphorus 0.222 Iron Balance l After being subjected to the same treatment as in Example I, the product'wasfound to have an essentially ferritic microstructureand a kockwell C hardness of 37. The stress rupture tests under 50,000 p. s. i. at 1200 F. showed a life of 52.6 hours with an elongation in one-inch gauge length of 6.4% and a reduction in area of 16.7%.

Example Vll.An alloy was prepared having the following analysis:

Percent Carbon 0.034 Manganese 0.33 Silicon 0.37 Chromium 1 1.02 Molybdenum 14.47 Phosphorus 0.218 Iron Balance When subjected to the same treatment as specified in Example I, the product was essentially ferritic in structure after annealing and aging and had a Rockwell C hardness of 54/56. Samples were subjected to stress rupture tests and the results recorded below:

Percent Percent Test Temp. Load, Elongation Reduc- Life in p. a. i. in one-inch tion in Hours gauge length Area Example VIII.--An alloy was prepared having the following analysis: 1

After being subjected to thesame treatment as in Example I, the product was found to have an essentially ferriticmicrostructurelandfalkockwell C hardness of 49/51. The stress rnpturetests under 60,000 p. s. Lat

4200' .F. showed a life of 119.4 hours with an elongation in one-inch gauge length of 2.8% and a reduction in area of 6.3%. l

Example IX.-An all y was prepared having the following analysis:

Percent 0.106 0.37 0.21 11.99

Carbon Manganese Silicon Chromium Tungsten Phosphorus 0.216 Iron Balance ,When subjected to the sametreatment as specified in Example I, the product was essentially ferritic in structure after annealing and aging. Samples weresubjected to stress rupture tests and the results recorded below:

Percent Reduc- Percent Elongation in one-inch gauge length Life in Test Temp. H

. OlllS tion in Area Example X.-An alloy was prepared having the following analysis:

t 1 Percent 0.026. 0.27 0.19 11.29 9.49

Carbon Manganese Silicon Chromium Molybdenum Phosphorus Iron Balance After being subjected to the same treatment as in Example I, the alloy was found to have an essentially ferritic microstructure. The stress rupture tests under 35,000 p. s. i. at 1200 F. showed a life of 1438 hours.

The terms and expressions which we have employed are used as terms of description and not of limitation, and we have no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but recognize that various modifications are possible within the scope of the invention claimed.

We claim: 4

l. A steel alloy capable of being age-hardened to show an essentially ferritic structure, said alloy having good corrosion resistance, ductility, impact strength and life at elevated temperatures, and said alloy containing up to about 0.10% carbon, silicon in an amount not more than 1%, manganese in an amount not more than 1%, 9-16.5% chromium, 647% of an element selected from the class consisting of molybdenum and tungsten, and the balance iron except for impurities usually associated with these elements.

2. A steel alloy having a structure when aged that is essentially ferritic and having a Rockwell C-scale hardness of above 25, said alloy containing up to about 0.10% carbon, silicon in an amount not more than 1%, manganese in an amount not more than 1%, 9-l6.5% chromium, 6-17% of an element selected from the class consisting of molybdenum and tungsten, and the balance principally iron.

3. A turbine part exposed to high temperature and stress, said part consisting of an essentially ferritic alloy having good corrosion resistance, ductility, impact strength and life at elevated temperatures and being readily machineable at room temperautre, said alloy containing up to about 0.10% carbon, silicon in an amount not more than, 1%, manganese in an amount not fmor'e' than 1%, 9-"l6.5 'chr omium 6 l7%"6fj%tii element selected from the class gcoris'istingi 'p',,.ni,c}lyb;

denurn and tungsten, and the balance principally iron.

'stantially all iron, said alloy having an essentially .:fer-

ritic'microstructure when aged. M

6. A steel alloy containing carbon up to 0.10%, silicon and manganese in an amount up to about 1.0% each, chromium 9-l6.5%, 6 -'17% of an element selected .from the class consisting of molybdenum "and I tungsten, up to 1.0% vanadium, and the 'balances'ub-' stantially all iron, said alloy having an essentially fer ritic microstructure when aged.

7. A steel alloy containingcar'bon up to 0.10%, silicon and manganese in an amount up to about 1.0% each, chromium 916.5%, '6-17% of an element se lected from the class consisting ofmolybdenum and tungsten, up to 0.5% titanium, and thebalance substantially all iron, said alloy having an essentially ferritic microstruoture when aged.

8. A steel alloy containing carbon up to 0.10%, silicon and manganese in an amount up to about 1.0%

each, chromium 946.5%, 6-17% of an element 'se' lected from the class consisting of molybdenum and tungsten; f "to 0.01% boron, and the balancesubs'tantiallyall :irohf'siidflalloy having anessentiaIIYferritic micro'st'ruct T,

ien aged. r h

9. steerandy containing carbon'up to QZ%, fsili con up to 1.0%, manganese up to 1.0%; bhi'o'miuin 'l0 14%, an element selected from the class consisting 9t j tungsten-and molybdenum 8-l4%, nickel up t o 610%, and the balance substantially all iron, said Janey having an essentially -ferritic 'microstructure when ager- 1;.

r 1, 10. A- steel alloy capable of being-annealed and aged 0.218%, boron to give an essentially -ferrit-ic structure, said alloy consistingessentially of carbon about 0.022%, silicon about 0.20%, manganese about 0.27%, chromium about 11.3%; molybdenum about 9. 9%, phosphorus about less than 0.000;5%, and the balance t ly.- .;it0n-. ,.1 Y

:11. A steel alloy capable of being jannealedandaged to give an essentially ferritic structure, said alloy con: sisting essentially of carbon about 0.034%, manganese about 033%, silicon about 0.35%, chromium about 0.194%, boron less t han0. 001% nickel about 407%,;

11.4%, molybdenum about 10%, phosphorus about and the balance substantially all iron.

ri is esswn't sfi s rth sa UNITED STATES PATENTS 2,737,455 Kir-kby-et al. Mar. 6, '1956 

1. A STEEL ALLOY CAPABLE OF BEING AGE-HARDENED TO SHOW AN ESSENTIALLY FERRITIC STRUCTURE, SAID ALLOY HAVING GOOD CORROSION RESISTANCE, DUCTILITY, IMPACT STRENGTH AND LIFE AT ELEVATED TEMPERATURES, AND SAID ALLOY CONTAINING UP TO ABOUT 0.10%, SILICON IN AN AMOUNT NOT MORE THAN 1%, MANGANESE IN AN AMOUNT NOT MORE THAN 1%, 9-16.5% CHROMIUM, 6-17% OF AN ELEMENT SELECTED FROM THE CLASS CONSISTING OF MOLYBENUM AND TUNGSTEN, AND THE BALANCE IRON EXCEPT FOR IMPURITIES USUALLY ASSOCIATED WITH THESE ELEMENTS. 